The present invention arose during continuing development efforts relating to the subject matter of U.S. application Ser. No. 07/168,932, filed Mar. 16, 1988, and U.S. Pat. Nos. 4,665,549, 4,677,676, 4,677,677, 4,736,431, incorporated herein by reference.
The present invention involves differential bandpass filtering of the error signal to a narrower range than the input signal to improve system performance by reducing the range of modeling away from the cut-off frequencies of the input signal where sharp bandpass filtering is otherwise required to minimize regions of instabilities due to rapid phase change near the cut-off frequencies of the bandpass filtering.